Taking a peek at the forums here and ProfitUI's eq2ui_mainhud_startbutton.xml, I see that the way it's achieving the greater light for the nightvision setting is by using r_flat_ambient.

The problem with this setting is that it sets the transparency layer of the graphics to be opaque black. This is really noticeable somewhere such as the Fens of Nathsar, where instead of a few wispy tree branches hanging down, with nightvision you have big opaque black slabs.

Is there a way to improve the ambient light WITHOUT changing that transparency layer?